1. Field of the Invention
The present invention is directed to optical metrology, and more particularly to a system for measuring tear film dynamics and material interaction of tears in the human eye using phase shifting interferometry.
2. Discussion of the Related Art
In the human eye, the tear film is distributed over the cornea to create a smooth surface. Since the largest refractive index difference in the eye occurs at the air-to-tear film interface, this surface contributes a majority of the eye's optical power. In addition to its optical properties, the tear film serves to lubricate the eye, and in general keep it in a healthy state.
When a person blinks, a new tear film is distributed on the cornea. After the blink, the tear film stabilizes. At this point in time, the tear film is as smooth as it will ever be. Essentially, this is in the optimal state for the tear film. If no blinking occurs, the tear film normally begins to breakup over a period of time ranging from about four (4) to about fifteen (15) seconds. During the breakup, the tear film becomes turbulent and begins to dry up in places. This may cause decreased visual acuity along with discomfort. Non-uniformity in the tear film may also lead to refraction errors caused by light scatter.
When a contact lens is placed on the eye, layers of tear film form both between the contact lens and the cornea and over the anterior contact lens surface. Proper distribution of the tear film is critical to achieving a comfortable lens fit and vision improvement, so lens materials must be designed to have a proper wetability. While some lens materials provide improvements such as increased oxygen permeability, their effects on tear film distribution are unknown. Presently, the lens must be tested in vivo during clinical trials where qualitative and only semi-quantitative tear film analysis methods, such as using fluorescein eye stain and slit lamp imaging, are used to evaluate tear film evolution and breakup. However, these methods lack the sensitivity and resolution to fully observe tear film dynamics. Therefore, a high resolution method of measuring tear film topography in vivo is desired.
In current practice, one method of tear film examination relies on instilling fluorescein in an eye and examining the fluorescing light with a slit lamp or similar instrument. In this method, the examination is completely subjective and no quantitative analysis of the tear film can be made. Furthermore, introducing a foreign element into the eye could alter the tear film itself.
Another current method involves using a corneal topographer where a ring or grid pattern is reflected off of the tear film and their deviations from the desired shape provide information about the tear film topography. These systems have low sensitivity and spatial resolution and are not capable of measuring relatively small artifacts in the tear film.
Some research has been done using shearing interferometry to measure the tear film topography. In this method, the wavefront reflected from the tear film is split and shifted in order to be interfered with itself. While this provides higher sensitivity and resolution than previously discussed methods, its sensitivity resolution and are less than what is required to provide early identification of tear film artifacts. Also, to our knowledge, no attempt has been made to convert the shearing interferometry data to an actual surface topography. Instead, Fourier analysis is performed on the fringe patterns and their changes used to quantify changes to the tear film.
Accordingly, there exists a need for developing a system and methodology for analyzing tear film dynamics in the human eye, in vivo, and then using this information to calculate the tear film's topographic surface profile with better accuracy and resolution than the previously described systems.